The present invention relates to a rare earth-based sintered permanent magnet and a permanent magnet synchronous motor therewith suitable for high-speed revolution as a motor of electric cars and a factory-automation motor.
As is well known many of compact motors built in electric and electronic instruments in the fields of acoustic and imaging technology are constructed by using rare earth-based permanent magnets. The reason therefor is that, since rare earth-based permanent magnets have very excellent magnetic properties such as coercive force and residual magnetic flux density as compared with other types of permanent magnets, a greatly increased freedom can be obtained in the design of electric motors by the use thereof enabling to construct a high-performance compact motor of small thickness which can be built in a compact-size instrument.
Since the rated power of these compact motors is mostly 100 watts or smaller, it is accepted heretofore that the eddy current and heat generation thereby in the rare earth-based permanent magnets have relatively small influences as the factors to cause a loss in the efficiency of motors as compared with other factors leading to a loss of the motor efficiency.
In contrast to the above mentioned situations heretofore in the permanent magnet motors, it is a trend in recent years that rare earth-based permanent magnets are used in much larger motors such as AC servomotors having a rated power of several hundreds watts to several tens kilowatts and DC brushless motors for driving electric cars having a rated power of up to several tens of kilowatts. Needless to say, rotors in many of these large motors require a rare earth-based permanent magnet much larger in size and capable of withstanding a high velocity revolution of the motor reaching 5000 rpm or even higher. As a result, the eddy current and heat generation thereby occurring in the rare earth-based permanent magnets, which present no particular problems in compact motors, can no longer be disregarded as a loss factor of the motor efficiency. Occurrence of eddy currents is also a serious problem in respect of motor control as in a rotor magnet which can be subject to application of a magnetic field in the reversed direction or rapid changes in the armature magnetic field. Accordingly, it is eagerly desired to urgently solve these problems, which can be disregarded in compact motors, in order for large-capacity motors with a rare earth-based magnet to go into orbit of practical application.
Rare earth-based permanent magnets in general have a volume resistivity in the order of 10xe2x88x924 ohmxc2x7cm which is substantially higher than that of many iron-based materials of which the volume resistivity is in the order of 10xe2x88x926 ohmxc2x7cm. A high resistivity of a magnet may be an advantageous factor for reducing the eddy currents. Different from iron-based materials from which a high-resistance structure can be constructed by utilizing thin plates suitable for punching works and lamination with intervention of insulating layers, however, a high-resistance structure cannot be formed of a rare earth-based permanent magnet material which must be used in a bulky form due to the relatively high brittleness of the material.
In the conventional synchronous motors built by utilizing a ferrite magnet, the eddy current generated in the magnet has absolutely no problem on the efficiency of the motor since ferrite magnets are inherently insulating as a material. Nevertheless, a sufficiently high efficiency suitable for practical use cannot be obtained in a large permanent magnet motor utilizing a ferrite magnet due to the low magnetic properties of ferrite magnets in general. The rare earth-based permanent magnets, on the other hand, belong to the class of metallic materials even though the resistivity thereof is relatively high as compared with other metallic materials for permanent magnets.
Under the above described situations including the application fields and conditions of use, accordingly, the most serious problem against a large motor using a rare earth-based permanent magnet is the eddy current generated in the magnet not only in respect of the decrease in the motor efficiency but also in respect of possible demagnetization of the magnet due to the temperature elevation by the heat generation caused by the eddy currents. Incidentally, no particular difference is noted relative to the eddy current loss or iron loss occurring in the rotor core of a large permanent magnet motor since the rotor core has a laminated structure of thin plates of an iron-based material or is a bulk core of an iron-based material even in a motor utilizing a rare earth-based permanent magnet.
The eddy current generated in a rare earth-based magnet can of course be decreased by utilizing a base material of the rare earth magnet alloy having a higher volume resistivity as in the ferrite-based permanent magnets. Alternatively, the eddy currents can be decreased in a so-called segment magnet which is an assemblage of a plurality of magnet segments adhesively bonded together with intervention of an insulating layer between adjacent segments. The former way involves a practically very difficult problem since the resistivity of the magnet material must be substantially increased without decreasing the inherent magnetic properties of the magnet.
Although, on the other hand, the latter way of segment magnets seems to be practical, the manufacturing cost of segment magnets is necessarily high because of the increase in the number of the process steps and a relatively low weight yield of the magnet material. Further, segment magnets are under a risk of relatively low corrosion resistance because a great difficulty is encountered in the surface treatment of the magnet segments for the formation of a highly corrosion-resistant insulating coating layer on the glueing surfaces of the segments. The idea for the use of magnet segments as such in the form of discrete small magnet pieces without glueing together is not successful because of the difficulty in obtaining high dimensional accuracy by mounting the magnet pieces on the proper positions of the rotor by resisting against the repulsive force between the magnet pieces.
Although no fundamental solution can be obtained for the problem of eddy currents in a rare earth-based permanent magnets per se, it would be a due direction of improvement to increase the heat resistance of the rare earth-based magnets with little demagnetization at an elevated temperature so that the magnet can be employed at an elevated temperature under little influences of the heat generated by the eddy currents. It is known, for example, that a Nd/Fe/B-based sintered magnet can be imparted with an increased coercive force by the addition of a certain additive element such as dysprosium to the base magnet alloy composition consequently leading to an improvement in the heat resistance of the magnet. Namely, a permanent magnet having a greatly increased coercive force at room temperature, when brought under an elevated temperature, can retain a coercive force sufficiently high to withstand demagnetization. Incidentally, Nd/Fe/B-based permanent magnets are required to have heat resistant against a temperature of 150xc2x0 C. in an electrical-equipment motor and 200xc2x0 C. in a driving motor of electric cars.
The above mentioned improvement in the coercive force of the Nd/Fe/B-based magnet, however, is not without problems because addition of an additive element to the alloy composition usually results in a decrease in the residual magnetization so that the magnetic flux density which can be taken out of the magnet is necessarily decreased. In addition, an increase in the material cost of the magnet is unavoidable by the use of an additional metallic element for the improvement of the heat resistance.
In view of the above described problems and disadvantages in the prior art, the inventor previously proposed a synchronous motor with a rare earth-based permanent magnet built therein, which is provided on one of the surfaces with a plurality of slits which have an effect of decreasing the effective cross sectional area available to eddy currents consequently contributing to a decrease of the eddy currents. Though effective for decreasing the eddy currents, the magnet block with slits necessarily leaves a bulk portion out of reach of the slits where no decreasing effect can be exhibited against generation of the eddy currents. In other words, the decreasing effect on the eddy currents is correlated with the depth of the slits formed in the magnet block. The incision depth of the slits formed in a magnet body cannot be too large because the mechanical strengths such as bending strength of the magnet block are greatly decreased by increasing the incision depth of the slits. Thus, the effect of slits formed in a magnet block for decreasing the eddy currents is directly limited by the tolerable depth of the slits.
As is described above, a permanent magnet synchronous motor of a kilowatt order or larger power utilizing a rare earth-based permanent magnet involves a serious problem of demagnetization of the magnet and a decrease in the efficiency of the motor under highspeed revolution due to the eddy currents generated in the magnet and the temperature elevation of the magnet as a consequence of the eddy currents.
The present invention accordingly has an object to provide a rare earth-based permanent magnet block which is little subject to the generation of eddy currents and also to provide a permanent magnet synchronous motor of which the rotor is constructed by utilizing the above mentioned rare earth-based permanent magnet so that the efficiency of the motor is little affected by the eddy currents generated in the permanent magnet of the rotor.
Thus, the present invention provides a rare earth-based sintered permanent magnet block in the form having an upper surface and a lower surface substantially in parallel to the upper surface, each surface being provided with at least one slit, of which the running direction of the slit in the upper surface and the running direction of the slit in the lower surface make an angle of at least 10xc2x0 within the plane of the upper and lower surfaces and the total of the largest incision depth of the slits in the upper surface and the largest incision depth of the slits in the lower surface is not smaller than but not larger than {fraction (5/3)} times of the distance between the upper and lower surfaces assuming that two or more slits are formed in each of the upper and lower surfaces.
The permanent magnet synchronous motor of the present invention comprises a rare earth-based sintered permanent magnet block as defined above arranged in such a disposition that the surface provided with the slits faces the armature of the motor.